NO171253B - PROCEDURE FOR AA TO MAKE A COMPOSITE METAL ARTICLE, AND COMPOSITE METAL ARTICLE MANUFACTURED ACCORDING TO THE PROCEDURE - Google Patents

Description

The invention relates to composite metal articles. In particular, the invention relates to articles of two different metals which are firmly bound together, one metal protecting the other

a manner required for a particular application.

A number of different methods have been proposed for the preparation of composite metal articles to enable the use of desirable properties of two different metals. Thus, articles of a metal with low corrosion resistance are often protected by hard-coating or cladding with a wear- or corrosion-resistant metal such as stainless steel. Alternatively, tough but easily machinable metals can be similarly protected by the application of a material which gives a composite article the required wear resistance. In the latter case-

le supports and holds the tough metal, a relatively fragile abrasion-resistant material that can break under impact stress, while it also enables machining and fixing of the composite article in a way that is only possible with difficulty for an article of abrasion-resistant material alone.

Hard coating by welding deposition of metal to provide a composite article finds extensive use,

but is relatively time-consuming, labour-intensive, relatively expensive and subject to several practical limitations. However, resort to hard coating is necessary in many applications due to the lack of an economical and/or practical alternative. Various alternative proposals are presented in the British patents 888404, 928928, 977207, 1053913, 1152370, 1247197

and 2,044,646 and in US Patents 3,279,006 and 3,342,564.

British Patent 888,404 suggests a process for compound steel products such as mild or low alloy steel and a stainless steel, and a melt of one of the steels is cast around a solid of the other steel. The solid second metal is cleaned mechanically or chemically before the casting process, while the casting is carried out under vacuum. However, it is made clear that no complete bonding is achieved by the casting process alone. The composite article must thus be hot-rolled to weld the two

steel together, as the bond is provided by hot rolling. The process thus suffers from the disadvantage that it must be carried out under vacuum, a method which is not well suited for many

production situations, while the need for hot rolling limits the choice of materials with which the process can be applied, as well as the shape of the resulting composite article.

British patent 928 928 relates to liners for grinding mills and points out the problems resulting from the liner being made entirely of an abrasion resistant material such as carbidic cast iron, either unalloyed or an alloyed cast iron such as white nickel-chromium cast iron. It thus proposes a composite lining of such material and a substrate of a softer and tougher metal or alloy produced by a double casting operation in which a first metal is cast, and the second metal is cast against the first metal. Apparently aware of the difficulty of obtaining a bond between a solid and a cast metal, and unable with a skirted cast iron to have recourse to hot rolling to overcome this difficulty, British Patent 928 928 teaches that the first metal, typically the carbidic cast irons, are only partially solidified when the other metal is cast against it.

British patent 928 928 recognizes the adverse consequences of oxidation on the surface of the first metal against which the second metal is to be cast. For this purpose, a mold is used to achieve rapid cooling of the first metal to a partially solidified state. To further counteract oxidation, however, a flux can be used to protect this surface, the flux being present in the mold before the casting of the first metal or added in liquid form together with the first metal.

Because the substrate is cast according to the proposal

in British patent 928 928, its properties will be inferior to those of a knurled substrate. The necessity for the first metal to be only partially solidified when the second metal is cast also constitutes a significant limitation. Thus, accurate temperature control is absolutely necessary because of rapid cooling of the first metal melt and the necessity of casting the second metal while the first is only partially solidified. Casting the second metal while the first is still too hot, i.e. still contains liquid, will result in mixing of the metals and loss of properties due to dilution; and if the first metal is too cold, a

satisfactory bond not likely. Furthermore, the process requires that two melts be available at the same time and at well-regulated temperatures, and while some foundries will be able to meet this need, the problem remains of coordinating casting from the two ladles required. There is also the practical problem of replenishing solidification shrinkage in the cast first metal with metal of the same composition. In the description in British patent 928 928, such shrinkage can only be replenished from the second metal, so that the first metal will eventually contain areas of different composition. The process in British patent 928,928 also requires the surface of the first metal to be horizontal, severely limiting the range of composite articles that can be produced. Furthermore, the second metal must be supplied horizontally over this surface so that the two melts are not mixed to a high degree; while the rate of flow of the second metal over this surface must be regulated so that the first metal is disturbed as little as possible, for the same reason.

British patent 977 207 proposes a process for seamless compound products, such as pipes or rods, in which the respective parts are of mild steel such as stainless steel and a mild steel. In this process, a component of one of the steels is heated under vacuum or in a non-oxidizing atmosphere, and while maintaining this environment, it is rapidly lowered into a melt of the other steel. The heating temperature for the component of the first steel must be at such a temperature that its surface, after immersion in the melt of the second steel, becomes a semi-molten or very viscous melt so that the two steels are welded together after cooling. The necessity of an operation under a vacuum or a non-oxidizing atmosphere is a serious limitation, which typically necessitates a closed vessel in which the process is carried out to preclude oxidation after heating the first component to near the melting point of the second metal. Furthermore, the process is again limited in the area of shape or forms of composite articles that can be produced. The process is also not suitable for use where the two metals have significantly different melting points.

The serious disadvantages of operating with a non-oxidizing atmosphere also apply to the similar descriptions in British patent 1 053 913 and 1 152 370. These patent documents differ significantly from each other as regards the composition of the respective wear-resistant materials; patent 1,053,913 suggests white chrome-boron cast iron containing molybdenum and vanadium, while patent 1,152,370 suggests nickel-boron cast iron containing molybdenum and vanadium. In both cases, the solid cast iron, in the form of crushed ingots and pellets, is enclosed in a container which prevents atmospheric oxidation and in which it is to provide a lining, and is heated in the container under an inert atmosphere so that it melts. The vessel is rotated so that the molten cast iron is distributed, and the vessel and the forge are then cooled. In addition to the disadvantage of the need for an inert atmosphere and the rotation of the container until the cast iron is solidified, the known technique according to British Patents 1,053,913 and 1,152,370 has other disadvantages. The container must necessarily have a melting point substantially above the melting point of the cast iron, as the heating of the container must be limited to a temperature below that at which distortion or deformation of the container will take place, especially when it is rotated. Furthermore, the prior art has serious limitations in relation to the shape of the resulting composite article, in view of the centrifugal distribution of the cast iron melt; and there is no indication of how the container with the higher melting point can in practice be provided with externally distributed cast iron.

British Patent 1,247,197 is largely equivalent to British Patents 1,053,913 and 1,152,370. It differs primarily in its use of eutectic Fe-C, plus higher melting alloy, to form the cast iron.

US Patents 3,342,564 and 3,297,006 respectively relate to a composite article and a method for its production in which a melt of a metal is cast to fill a mold containing a solid second metal. Again, a vacuum or non-oxidizing atmosphere is necessary, due to the second metal being preheated to an elevated temperature so that melting of its surface occurs when casting the first metal, and the need to protect the second metal from oxidation.

Finally, British patent 2,044,646 suggests hot-welding a mild steel and a martensitic white cast iron. The welding can be achieved by casting the white iron on/to a plate of mild steel, the latter possibly being preheated. Alternatively, the cast iron can be cast first, and while it is still hot, mild steel is cast against it. In the first of these alternatives, hot welding is likely only if surface melting of the mild steel takes place, a situation which is not suggested by the freedom of choice inherent in possibly preheating the mild steel. Furthermore, oxidation of the mild steel takes place to such an extent that a satisfactory bond between the mild steel and the cast iron is difficult to achieve even with melting of the surface of the mild steel. Similar considerations apply in the second case, except that the oxidation is on the cast iron during its cooling. It is in fact only by mechanical separation resulting from perforations or the like in one metal, against which the other is cast, that the two metals are likely to be satisfactorily joined. Such interlocking, however, eliminates the advantage of a mild steel substrate in protecting brittle cast iron under impact stress, as the interlocking gives rise to localized stress concentration in the cast iron.

The present invention seeks to provide an improved composite metal article and a method for its manufacture which is better suited to simple foundry practice and which enables a wider choice of metals.

The invention provides a method for forming a composite metal article having a first component 14 of a ferrous metal and a second component that is cast over a substantially oxide-free bonding surface of the first component 14, wherein the first component 14 is placed in a mold 10 such that together with the mold 10, it delimits a cavity where a melt is poured to provide the second component, the first component 14 being heated to a preheating temperature by the first component being at least partially heated in the mold 10 before the melt is poured in, characterized in that the bonding surface is provided with a flux coating, at least before the first component is heated in the mold (10),

the first component (14) is heated to a preheating temperature of from 350°C to 800°C,

the alloy which will form the second component is selected from the group comprising ferrous metals and cobalt-based metals,

the melt is poured at a superheated temperature and in such a manner that the melt flows over the bonding surface to displace the flux coating from the bonding surface and to

moisten the bonding surface,

the temperature designated as superheated temperature,

is significantly above the preheating temperature, whereby the melting causes the temperature of the bonding surface to increase so that an initial temperature equilibrium is achieved between the surface and the melting, and essentially instantaneously an interface temperature between them which is at least equal to the liquidus temperature of the melt, so that after solidification of the melting, a bond is achieved between the resulting second component and the first component (14) without a fusion of the bonding surfaces taking place.

The required bond, which is substantially free of a melt layer, is achieved if the surface of the first component is wetted by the melt which is to form the second component. Such wetting of the surface has been found to take place if: The invention also relates to a composite metal article produced according to the method in claim 1, which article has a first component (14) and a second component, where the second component is cast against a surface of the first component (14), which article is characterized by a diffusion bond between said components obtained after solidification of the melt which provides said second component mainly without melting of said surface. (a) a favorable surface energy relationship exists between the surface of the first component and the melt, - a condition obtained if the surface is substantially free of oxide contamination but hindered by such contamination, and (b) the first component has a relatively high melting point and its surface, with the melt cast against it, attains a sufficiently high temperature, most preferably a temperature equal to or higher than the liquidus temperature of the melt.

The bond is generally sharply defined, but typically exhibits some diffusion between the components in the solid state.

While a molten layer resulting from melting of the first layer is essentially avoided, the bond can further be characterized by micro-dissolution, as opposed to melting, of the first component in the melt before solidification of the latter. Some epitaxial growth from the surface of the first component can also take place, although this has not been seen to characterize the bond to a visible degree.

It has thus been found that the achievement of a good bond when casting a melt of a metal against a solid component depends, among other things, on the temperature prevailing at the surface of the solid component against which the melt is cast, and also the absence of oxidation of this surface. In general, in accordance with known technology, attempts have been made to protect against oxidation by using a vacuum or non-oxidizing atmosphere, a vacuum having generally been preferred. Practically speaking, however, casting under vacuum is not well suited for industrial casting practice and necessitates expensive equipment. Especially in repetitive casting operations, it also significantly increases the production time. Similar comments apply to casting under a non-oxidizing atmosphere, as casting under such an atmosphere must be carried out in a closed container similar to that required for casting under vacuum, to provide adequate protection of the first component. That is to say, especially when the solid first component is heated, which is necessary for a good bond, that the precautions necessary to protect the surface against oxidation increase with the temperature, and it is necessary that the melt for the second component is cast against this

surface essentially in the absence of oxide on the surface.

It has been found that a good bond is obtained if the surface of the first component is cleaned to remove any oxide film and then protected, until the melt for the second component is cast against it, by a film of a suitable flux. A number of different fluxes can be used, while these can be applied in different ways. However, most preferred is

an active flux in that it not only prevents

oxidation of the surface of the first component, but also cleans this surface of any oxide contamination that remains, or is present, after cleaning this surface. Suitable fluxes include "Comweld Bronze Flux", which has a melting point of approx. 635°C and contains 84% boric acid and 7% sodium metaborate, "Liquid Air Formula 305 Flux" (650°C, 65% boric acid, 30% anhydrous borax) and "CIG G.P. Silver Brazing Flux"

(485°C and containing boric acid plus borates, fluorides and fluoro-borates) , Less active fluxes, such as anhydrous. borax (740°C), which only provides a protective film but does not remove existing oxide contamination on the surface, can also be used provided that such contamination is first removed mechanically or chemically.

As mentioned above, the temperature that prevails at the surface of the solid component against which the forging is cast is an important parameter. By this is meant the temperature at the interface between the components after casting the forge. This parameter, although important, is secondary to the need for the surface of the solid component to be free of oxide, as achieving an otherwise sufficient interface temperature will not give a good bond if this surface is oxidized.

The interface temperature that is achieved depends on several factors. These include the temperature to which the solid component is preheated, the degree of superheating of the forging when it is cast, the area of the surface of the solid component against which the forging is cast, and the mass of the solid and cast components. When the respective metals in these components are different, additional variables also include the respective thermal conductivity, specific heat and density of these metals. Despite the complex interrelationships that these parameters give rise to, it has however been found that a satisfactory bond can be achieved when the solid component is preheated to a temperature of at least approx. 350°C. The solid component is preferably preheated to a temperature of at least 500°C.

It is strongly preferred that the temperature at which

the solid component is preheated and the degree of overheating of the melt is such that, after casting the melt, an interface temperature equal to or higher than the liquidus temperature of the melt is achieved. It has been found that the essentially instantaneous interface temperature is not simply the arithmetic mean of the preheating and melting temperatures, taking into account where necessary differences in thermal conductivity, specific heat and density, as would be expected. Such an arithmetic means actually results in an erroneously low determination of essentially instantaneous interface temperature, as the calculation assumes that the heat transfer from the melt to the solid component occurs exclusively by conduction. Calculation of the Nusselt number for the smelting shows that heat transfer by convection in the smelting is also important, and when this is taken into account, it shows that the mainly instantaneous interface temperature can be from 150°C to 200°C higher than the arithmetic mean of the the heating temperature of the solid component and the melting temperature.

The requirement that an interface temperature equal to or above the liquidus temperature of the melt should be achieved means that the invention is primarily applicable when the solid first component has a melting range that begins at a temperature at least equal to the liquidus temperature of the melt for the provision of the second component. Furthermore, it must be remembered that while in the previous section reference is made to the essentially instantaneous interface temperature, this reference is made by way of example. This means that the required interface temperature does not have to

be achieved instantaneously and may be briefly delayed such as due to a temperature gradient with the first component. It should also be noted that the invention can be used when the melt which is to provide the second component is of essentially the same composition as the first component, the first and the second component thus having essentially the same melting point

area. In such a case, it remains desirable that the surface of the first component against which the melt is cast still achieves, after casting the melt, a temperature at least equal to the liquidus temperature of the melt, but that the body of the first component acts as a heat sink which rapidly reduces this surface temperature before significant melting of the surface takes place. Likewise, the invention can be used when the first component has a melting range that begins below the melting range of the material for the second component, provided that such rapid cooling can prevent significant surface melting of the first component; however, such a first component with a lower melting range is not preferred.

Achieving a sufficient interface temperature is accomplished by a balance between preheating of the first component and the degree of superheating of the melt to provide the second component. Preheating is preferably to a temperature above 350°C, more preferably to at least 500°C. The melt is preferably superheated to a temperature of at least 200°C, more preferably at least 250°C, above its liquidus temperature.

The use of a flux and the achievement of a sufficient interface temperature enable the achievement of a good bond between similar metals and also between dissimilar metals. We have found that these factors enable such bonding to be achieved when casting a stainless steel against a mild steel or an alloy steel such as a stainless steel. Likewise, a good bond has also been found to be achieved by casting a cast iron, e.g. a white cast iron such as a white chrome cast iron, versus a mild steel, an alloy steel such as a stainless steel, or cast iron such as a white cast iron. Furthermore, cobalt-based alloys can likewise be cast against a mild steel or an alloy steel to achieve a good bond between them.

Stainless steels with which very good results can be obtained, either as the solid first component or the cast second component, include steels such as austenitic grades equivalent to "AISI" 316 or "Australian Standard 2074-H6A", with a maximum of 0.08 wt. % carbon, 18-21 wt% chromium, 10-12 wt% nickel and 2-3 wt% molybdenum, the rest being mainly iron. "AISI" 304 stainless steel, with a maximum of 0.08 wt% carbon, 18-21 wt% chromium, 8-11 wt% nickel, and the remainder mainly iron, can also be used.

Suitable cobalt-based alloys include those with compositions of the type (Co,Cr)7C3~ carbides in a eutectic structure and a kneadable matrix, such as materials comprising 28-31 wt% chromium, 3.5-5.5 wt% tungsten, maximum 3.0 wt% iron, maximum 3.0 wt% nickel, maximum 2.0 wt% manganese, maximum 2.0 wt% silicon, maximum 1.5 wt% molybdenum, 0.9-1.4 wt % carbon and the rest mainly cobalt. A cobalt-based alloy with the nominal composition of 29 wt% chromium, 6.3 wt% tungsten, 2.9 wt% iron, 9.0 wt% nickel, 1.0 wt% carbon and the balance mainly cobalt has also been found to be suitable.

Cast iron used as the second component includes white chrome iron, with hypo- or hyper-eutectic composition. For these, the carbon content can be in the range from 2.0 to 5.0% by weight, while the chromium content can be substantially above chromium additives used to reduce graphitization in cast iron. The chromium content is preferably above 14% by weight and may be as high as from 25 to 30% by weight. Conventional alloying elements normally used in white chrome iron may be present in the component of this material. Particular white chromium irons found to be suitable in the present invention include: (a) Australian Standard 2027 grade Cr 15 Mo 3 , cast iron with 2.4-3.6 wt% Cr 15 Mo 3 carbon, 0.5- 1.5 wt.% manganese, maximum 1.0 wt.% silicon, 14-17 wt.% chromium and 1.5-3.5 wt.% molybdenum, the rest apart from incidental impurities being iron. (b) Australian Standard 2027 grade Cr 27 cast iron with 2.3-3.0 wt% carbon, 0.5-1.5 wt% manganese, maximum 1.0 wt% silicon, 23-30 wt% chromium and maximum 1 .5% by weight molybdenum, the rest apart from incidental impurities being iron. (c) Austenitic chromium carbide iron with 2.5-4.5 wt% carbon, 2.5-3.5 wt% manganese, maximum 1.0 wt% silicon, 25-29 wt% chromium and 0.5 -1.5% by weight molybdenum, the rest apart from incidental impurities being iron. (d) Complex chromium carbide iron with 4.0-5.0 wt% carbon, maximum 1.0 wt% manganese, 0.5-1.5 wt% silicon, 18-25 wt% chromium, 5.0 -7.0% by weight molybdenum, 0.5-1.5% by weight vanadium, 5.0-10.0% by weight niobium and 1.0-5.0% by weight tungsten, the rest apart from incidental impurities being iron. (e) Complex chromium carbide iron with 3.5-4.5 wt% carbon, maximum 1.0 wt% manganese, 0.5-1.5 wt% silicon, 23-30 wt% chromium, 0.7 -1.1% by weight molybdenum, 0.3-0.5% by weight vanadium, 7.0-9.0% by weight niobium and 0.2-0.5% by weight nickel, the rest apart from incidental impurities being iron.

The specific listed castable metals which are suitable for use in the invention as the second component will be recognized as surface cladding materials which are conventionally applied by hard coating by weld deposition. Such metals are typically applied to provide wear-resistant surface coatings. However, in the case of stainless steels, which can provide abrasion resistance at low or medium temperatures, the purpose of its use in a composite article may be wholly or partly to achieve corrosion resistance for the article's second component. While the invention primarily concerns composite articles which have abrasion resistance by suitable selection of the metal for one component, it thus also concerns articles for use in environments other than those in which abrasion resistance is required. As indicated by the ability to cast e.g. a cast iron against a cast iron, the composite article according to the invention can also be used for the reconstruction of a worn or damaged part of an article, the first and the second component in this case being of essentially the same or similar composition if required. In such rebuilding, the worn or damaged portion of an article may be machined, if required, to provide a more regular surface against which to cast a melt of rebuilding metal. However, such machining need not be necessary to achieve a good bond, provided that an oxide-free surface is available against which the forging is to be cast.

The solid first component can be preheated exclusively in the mold, although it can partly be preheated before it is placed in the mold, while the type of mold used can vary with the nature of the preheating. When heating only takes place in the mold, the preheating can be done by induction coils or by flame heating. For partial heating before placement in the mold, resistance, induction or flame heating can be used or, alternatively, the solid first component can be preheated in a muffle furnace or in an induction furnace. What is important, in each case, is that at least the surface of the component against which the forge for the other component is to be cast is thoroughly cleaned mechanically and/or chemically and protected, - before preheating to a temperature of which reoxidation will take place, with a suitable flux. In such cases, the flux is normally used as a slurry, such that the flux is painted onto at least this surface of the solid first component. Alternatively, the flux can be sprinkled on the surface in powder form, provided, when preheating is then to be with a flame, that the surface has been partially heated to a temperature at which the flux becomes sticky. Especially when the surface of the first component against which the melt is to be cast is of a complex shape, the flux can alternatively be applied by dipping the first component in a bath of molten flux. With each of these methods of applying the flux, the first component can be stored, after being coated with the flux, until it is needed for preheating. Alternatively, the component can be preheated immediately after the flux has been applied.

When the flux is applied by dipping the solid first component in a bath of molten flux, a variant of the above-described preheating methods can be used. In this case, the preheating can be carried out in part by immersing the solid first component in the bath of molten flux. The component can then be transferred to the mold and, after further induction or flame heating or after being allowed to cool to the required preheating temperature, the ingot is cast to provide the second component.

When preheating of the solid first component occurs at least partially by flame heating, the component may be placed in a mold that restricts a firing opening that enables a heating flame to project into the mold cavity and over this component, the flame preheating the component and also heating the mold . If not essential, a reducing flame can be used to maintain a reducing atmosphere in the mold to thereby further prevent oxidation of the surface of the first component. The flame may be provided by a burner near the combustion opening to generate the reducing flame.

The mold for use in flame heating can be constructed in parts that are separable. The parts can be separated by opposite side walls, and at one end of these walls the combustion opening can be defined, with an outlet opening for combustion exhaust gases from the flame defined at the other ends of the side walls. The side walls can be separable from the form parts or each can be one with the same or a respective form part. Preferably, an inlet channel is placed at the firing opening for guiding the flame into the interior of the mold. When the first component has an extended surface over which the forging is to be cast, such as a main surface of a flat plate substrate, the width of the firing opening in a direction parallel to this surface may be substantially equal to the dimension of the substrate surface in this direction. The channel can have opposite side walls that diverge towards the burning opening so that the reducing flame is spread out in a fan-like fashion to a width that extends over essentially the entire surface of the substrate against which the melt is to be cast. Furthermore, the channel may have top and bottom walls converging towards the combustion opening to assist in achieving such flame width. The channel can be separable from the mold, in one with a mold part or longitudinally separable from the mold, in one with mold part or longitudinally separable with a part thereof in one with each mold part.

The flame heating can be maintained until casting

of the forging is complete. After adding the melt and before it has solidified, the burner can be adjusted so that it produces a warmer, somewhat lean flame. Solidification of the top surface of the melt can be delayed by such a lean flame, so that the melt solidifies preferentially from the interface between the melt and the first component rather than simultaneously from this interface and the top surface. Such solidification can also minimize voiding due to shrinkage in the unfed cast metal.

With such flame preheating, the casting arrangement is most appropriate so that the melt is distributed quickly over all parts of the surface of the first component on which it is to be cast, and so that turbulence in the melt is maximized. Such rapid distribution and turbulence promotes heat transfer and a high, uniform temperature at the interface between the supplied melt and the surface-first component. Rapid distribution and turbulence also facilitate the breaking up and removal of enliver oxide film on the forge. It will also remove any residual oxide film on this surface, although reliance on this effect without prior cleaning and the use of a flux gives a clearly inferior bond.

Rapid distribution of the melt over the substrate surface of the first component and turbulence in the melt can be generated by a mold having a pouring basin in which the melt is received, and from which the melt flows via a plurality of runs whose outlet is arranged above this surface. This arrangement functions to evenly and simultaneously pour the melt onto/to all areas of the surface, which reduces the distance the melt must flow and helps achieve a high and uniform temperature at the interface between the melt and the first component. The arrangement also increases turbulence in the melt above, and facilitates wetting of, this surface.

An advantage of a reducing flame during such preheating of the first component is that it counteracts any tendency to oxidation of the melt due to its rapid distribution and turbulence. This turbulence can also cause erosion, by localized macro-dissolution of metal of the first component, at points where the melt hits the surface of this component.

It may therefore be advantageous to use an arrangement for casting the forge which establishes a substantially non-turbulent, progressive mold filling. In such an arrangement, the invention uses a mold which has a horizontally projecting inlet which causes the melt to enter a mold cavity in a plane essentially parallel to, and slightly above, the surface of the first component on which the melt is to be cast. This enables the melt to proceed in a substantially non-turbulent flow over this surface, with a minimum of flow splitting, whereby oxidation of the melt is inhibited. The exposure of fresh, non-oxidized metal of the forge to an oxidizing environment is thus minimized.

The location of the inlet is most appropriate so that the first melt entering the mold flows over the surface of the preheated first component and further heats this surface. Then incoming liquid metal displaces the first metal that entered the mold cavity, which ensures that as much heat as possible is supplied to the surface before solidification begins. Just before casting, the mold cavity can be closed with a mold top box, as the molten metal is poured into the cavity through a vertical downspout and a horizontal run system. For small castings, this system allows the casting of several castings in the same casting box from a single vertical downspout leading to separate inlets for each casting. This molding practice can be used to produce a bonding interface on a horizontal, inclined or even vertical surface of the first component.

In this arrangement, which provides mainly non-turbulent flow of the melt in the mold, flame heating can again be used. In this case, however, it is necessary to place the first component (which may be partially preheated) in the lower case part of the mold and, before placing the upper case part of the mold, to perform flame heating from above.

As an alternative, the mold can be completely assembled and preheating is carried out or completed in this by induction heating.

When flame heating is used, it is preferred that the flux is applied by dipping into a melt of the flux or by painting a slurry of the flux. If, as an alternative, it is required to apply the flux as a powder, it is preferred that the first component is heated slightly to 150-200°C, such as in a muffle furnace, so that the flux becomes sticky and is not blown from the surface of the first component of the heating flame.

When the flux is applied by dipping the first component in a bath of molten flux, the flux is applied at least over the surface of the component against which the melt is to be cast. Preferably, the component is immersed in the bath so that it is completely covered with flux and is also at least partially preheated in this bath. When a flux coating is provided, the first component is then placed in a mold, and a melt which is to provide the second component is poured into the mold so that the melt flows over the surface of the first component. The first component is preferably suspended in the bath of molten flux until its temperature exceeds the melting point of the flux. The component is then removed from the flux bath with a coating of a thin, adherent layer of the flux. The melting displaces the thin flux coating, as this is melted again if necessary, whereby the clean surface of the first component is exposed so that wetting and bonding take place. Obviously, the flux used must have a melting point that is sufficiently low to allow rapid remelting of the flux if it has solidified at the time the melt is poured into the mold. At the same time, the molten flux must be able to withstand temperatures sufficiently high that the steel substrate can be sufficiently preheated. A sufficient temperature can be achieved with several fluxes during the suspension, or dipping, of the first component in the bath of molten flux. However, when in particular the temperature of the flux bath is insufficient for this, or when the heat loss from the first component between the formation of the flux coating and the casting of the melt is too great, the first component is further preheated in the mold, such as by induction heating or flame heating.

For further explanation of the invention, the attached drawings shall be described:

Fig. 1 shows, in vertical section, an oven suitable for use in

a first embodiment of the invention.

Fig. 2 is a horizontal section taken along the line II-II on

fig. 1.

Fig. 3 is a perspective view of a mold pattern suitable

for the production of a mold component of an oven as shown in fig. 1 and 2.

Fig. 4 shows a flow diagram illustrating the fabrication

of composite metal articles in a second embodiment of the invention.

Fig. 5 shows a flow diagram illustrating a third embodiment of the invention.

Reference is made to fig. 1 and 2. A mold 10, formed from a bound sand mixture, has a lower mold part 12 in which is placed a ductile first component or substrate 14, on which a wear-resistant component is to be cast. A layer 16 of insulating material of ceramic fibers insulates the underside of the substrate 14 from the mold part 12, while a layer 18 of such material lines the side walls of the part 12 around and above the substrate 14. The mold 10 also has an upper part 20 above the part 12 and separated from this of opposing bricks 22. The space that the bricks 22 provide between the parts 12,20 is such that a transverse passage 24 is formed through the mold 10. Above one end of the passage 24, the mold is provided with an inlet channel 26, as the connecting between the latter and the passage 24 limits a burning opening 28. A burner 30, which can for example be suitable for gas or oil, is placed near the outer end of the channel 26

for generating a flame for preheating the substrate 14 and the mold parts 12,20.

The channel 26 has side walls 32 which diverge from the outer end to the burning opening 28. This arrangement causes the flame from the burner 30 to spread in a fan-like manner horizontally over substantially the entire width of the opening 28 and, within the mold 10, to pass through the passage 24 over substantially the entire upper surface of the first component or the substrate 14. The upper and lower walls 34, 35 converge to the opening 28 and thus contribute to the achievement of such flame width in the mold 10. Most expediently, the flame extends through the end of the passage 24 which is furthest from the opening 28, as combustion gases also flow out from this further end.

The upper part 20 of the mold has a section 36 with a filling pool 37 for supplying the melt of wear-resistant metal to be cast on the upper surface of the substrate 14. From the pool 37, the melt can flow under the action of gravity through a throat 38, along side runs 39 and through the various downspouts 40 in part 20. The lower ends of the downspouts 40 are distributed horizontally, so that the melt is filled evenly and simultaneously to all areas of the upper surface of the substrate 14.

Figure 3 shows a mold pattern for use in the manufacture of the upper part 20 of a mold corresponding to that in Figures 1 and 2. In Figure 3, corresponding parts are shown with the same reference number with symbols.

Castings made in a mold as shown in Figures 1 and 2 included steel substrates measuring 300 mm x 300 mm and 10 mm thick. The steel plates were inserted into the lower form part with insulation under and around the plates as described above. The molds were planed, flux was showered onto the steel to cover its upper surface, the mold was built up in the manner described, and the mold was initially slightly heated to make the flux sticky and adhere to the surface. Two sizes of castings were produced using a high chromium white cast iron, one type having 40 mm overlay on 10 mm steel plate, the other having 20 mm on 10 mm.

For the 4:1 ratio castings, the substrate was preheated using the burner which produced a reducing flame in the mold, and 30 kg of high chromium cast iron was cast at a temperature of approx. 1600°C poured into the filling basin. The surface of the iron was kept floating for approx. 8 minutes, and the burner was then shut off. A thermocouple against the bottom surface of the substrate reached a temperature of 12 50°C approx. 2 minutes after filling. Ultrasonic measurement indicated 100% bond, which was then confirmed by surface grinding the edges and by a diagonal cut through the casting, as well as. when removing 50 mm diameter cores by electrical discharge machining. The bond was free of any melt layer due to melting of the steel.

For the castings with a ratio of 2:1, the substrate was preheated, and 15 kg of the iron was filled in at a temperature of approx. 1600°C. The surface of the white iron could not be kept liquid as long as with the castings with a ratio of 4:1, but was liquid for approx. 5 minutes. The thermocouple towards the bottom of the plate reached 1115°C approx. 3 minutes after filling. For castings of this size, good bonding is again achieved over the entire interface between the substrate and the cast metal.

In addition to the castings described above, a number of further castings were produced of 200 mm x 50 mm x 10

mm steel substrates. In this case, the most appropriate mold was found to be a funnel shape with a long, narrow slit at the bottom. The gap extended the full length of the substrate and was narrow enough for the liquid iron to flow out from its full length at the same time. With a preheat of 350°C and an iron melt filling temperature of 1570°C, bonding was achieved over more than 95% of the total area. By increasing the mold heating temperature, bonding over 100% of the area can easily be achieved with substrates of this size.

The described castings have been shown to give complete bond on 300 mm x 300 mm x 10 mm mild steel test plates with white iron to steel ratios of 4:1 and 2:1. Higher and lower ratios are possible; the lower ratios depend in part on the substrate thickness and the rate of heat loss from the metal for optimal bonding.

Inherent in the invention is a high degree of freedom with regard to the geometric shape of the substrate and the finished article. The invention has significant advantages over other methods in that it enables the direct casting of hard, wear-resistant metals, such as white iron with a high chromium content, onto ductile steel substrates. The finished article can combine the well-documented wear qualities of, for example, white iron with the good mechanical strength and toughness, machining properties and weldability of low-carbon steel. The direct metallurgical bond between the white iron and the steel results in very high bond strength. The invention is particularly suitable for the production of hard coating layers with a thickness greater than those that can conveniently be deposited by T'ed welding processes.

The temperature to which the substrate is preheated can vary considerably. The temperature is limited by the need to prevent oxidation, the melting point of the substrate material, the need to minimize grain growth and the type of flux. Within these limits, a high preheating temperature is advantageous. The minimum preheating temperature will depend on the thickness ratio between the cast component and the substrate and on the size and shape of the components. For the above-mentioned 4:1 castings, a preheat temperature of 500°C was found to be just sufficient, while a minimum preheat of 600°C was found to be necessary for 2:1 castings.

An important parameter is the temperature at the interface between the casting melt and the substrate. This enables a reduction in the melting temperature with a corresponding increase in the substrate pre-heating temperature and vice versa. However, it is preferred that the melt is superheated sufficiently to allow any flux and any loosened glow scale to rise to the surface of the casting melt, and that the required interface temperature is achieved for a satisfactory bond between the substrate and the cast component. For all casting alloys, is

superheating by at least 200°C above the liquidus temperature preferred, most preferably at least 250°C above this temperature,

for achieving the required interface temperature during casting.

Especially with the flux provided over the substrate surface on which the melt is to be cast, the reducing flame need only provide a weak reducing atmosphere over this surface during preheating. For such an atmosphere, a flame provided at an air deficit of between 5% and 10% can be used.

Reference is made to Figure 4, where A shows an underside view of the upper case part 50 of the mold 52, and a ground plan of the lower case part 54 of this. In each of several mold cavities 56, there is a chamfered substrate 58, the upper surface of each of which is painted with a flux slurry. As shown at B, the substrates 58 are preheated from above with a flame, before placement of the upper case part 50, using a reflector 60 to facilitate the preheating. As shown at C, the upper box part then becomes 50

set in place, and a melt to be cast against the upper surface of each substrate is poured into the mold via top case opening 62. The melt flows horizontally via inlet 64, to each cavity 56, and flows along each substrate 58 over the full width of each . As indicated by D, they become the resultant

composite articles 66 taken out and then sanded in the normal way.

A method as illustrated in figure 4 has been used for the production of different sizes of hammer tips for use in hammer mills for unraveling heritage sugar cane. The hammer tips were made with substrates of mild steel and a cladding bonded to these of white cast iron with a high chromium content. The dimensions of the hammer tips produced have been as follows:

Sinking boxes have been used in the manufacture of hammer tips to ensure that fully satisfactory castings were formed. In these hammerhead types, significant chamfers have been machined into the substrates prior to casting, with the aim of enabling the manufacture of hammerheads with a more complete coverage of wear-resistant alloy on the working surface than has previously been possible with brazed composites. For these hammer tips, pre-machined substrates have also been used, in which drilled and threaded holes, which are required for the subsequent attachment of the hammer tip to the hammer head, have been formed before the production of the composite. The threaded holes have been protected with threaded metal inserts during the casting operation. The flexibility inherent in being able to use pre-machined substrates in this way has solved the problems associated with drilling and threading blind holes in an already bonded composite.

The hammer tips were found to be characterized by good diffusion bonding using casting temperatures comparable to those discussed in connection with Figures 1 to 3.

The bonds were diffusion bonds that did not show any melting layer as a result of melting of the substrate surfaces.

Reference is made to figure 5, where an oven 70 is shown at A

which provides a bath of molten flux 72 in which a tubular steel component 74 is immersed. The latter is preheated to a required temperature in the flux 72. As shown at B

and C, the heated component 74, coated with flux, is removed from the furnace 70, and after draining of excess flux, the component 74 is immersed in the lower case half 76 of a mold where further preheating is carried out, and the upper case

half 78 of the latter is put in place. In the arrangement shown, the mold includes a core 80 which projects axially through the component 74 and leaves an annular cavity 82 between the core 80 and the inner surface of the component 74. With the upper case half 78 in place as shown at D, a melt of superheated metal is cast which at E, via the upper case opening 84, whereby the cavity is filled.

Experiments with the above-described "Liquid Air" flux (m.p. 650°C) have been carried out in a procedure essentially as described in connection with Figure 5, using steel substrates comprising: (a) 200 mm long x 50 mm wide x 10 mm thick, for which bonding has been achieved with cast overlay thicknesses of 40 mm, 30 mm and 20 mm (ie 4:1, 3:1 and 2:1 casting ratios); and (b) 80 mm square x 25 mm thick, for which a good bond has been achieved with a thickness of cast overlay of 25 mm (i.e.

1:1 casting ratio).

It has been found that the flux layer that adheres to the substrate after it has been removed from the molten flux partial bath is relatively thick, and that mechanical scraping away most of this adhering flux leaves only a very thin layer , gave a better bond. A flux with a lower melting point can be used and has the advantage that there is more fluid at the required working temperature, whereby it drains better after removal of the substrate, and is also more easily melted again during casting. In the latter respect, however, it should be noted that it is not necessary for the flux to solidify between the removal of the substrate from the bath and the casting of the melt or the application of a flame or other preheating. The use of a flux with a lower melting point also facilitates the production of articles with an even smaller casting ratio than described herein.

While the articles described here are of planar form, it should be noted that the invention can be used to provide articles of a variety of different shapes. Thus, the invention can be used in the production of, for example, cylindrical articles which have a wear-resistant material cast on their inner and/or outer surface, curved elbows, T-pieces and the like. Representative additional composite articles that further exemplify the flexibility and range of possibilities of the present invention are set forth in the following table, in which: Method I denotes manufacture according to procedures described in conjunction with Figures 1 through 3, and

Methods II and III denote production in accordance with fig. 4 and fig. 5.

In all of the examples described in the table, good bonds were achieved in each case. It was found that the achievement of a good bond was relatively insensitive to the choice of flux or preheating method in either case. The preheating of the substrate component was generally to a temperature of approx. 800°C, and the melt was filled at one temperature of approx. 1600°C for all alloys. The above-mentioned "CIG Silver Brazing Flux" and "Liquid Air 305 Flux" were both found to be particularly suitable, especially in Method III.

The bond obtained with the present invention was found to be of good strength. This is illustrated for a composite article comprising AISI 316 stainless steel cast against and bonded to mild steel. For this article, bond strengths of approx. 44 0 MPa obtained with test specimens machined to have a minimum cross-section at the bond zone. Furthermore, with this article, a tensile strength of approx. 420 MPa for a test piece with 56 mm parallel length, with the bond approx. halfway along this length; the total extension of the 50 mm measuring length was 32%. For articles in which the cast metal component is brittle, the bond has been found to be stronger than the cast metal component of the article. Thus, with hypo-eutectic white chrome iron cast against and bonded to mild steel, bending tests showed that fracture cracks passed through the white iron and not the bonding zone.

Claims (28)

1. Method of forming a composite metal article having a first component (14) of a ferrous metal and a second component that is cast over a substantially oxide-free bonding surface of the first component (14), wherein the first component (14) is placed in a mold (10) so that together with the mold (10) it defines a cavity where a melt is poured to provide the second component, the first component (14) being heated to a preheating temperature by the first component being at least partially heated in the mold (10) before the batter is poured in, characterized by that the bonding surface is provided with a flux coating, i at least before the first component is heated in the mold (10), the first component (14) is heated to a preheat temperature from 350°C to 800°C, the forging that will form the second component is selected from the group comprising ferrous metals and cobalt-based metals, the melt is poured at a superheated temperature and on a in such a way that the flux flows over the bonding surface to displace the flux coating from the bonding surface and to wet the bonding surface, the temperature designated as superheated temperature, is significantly above the preheating temperature, whereby the melting causes the temperature of the bonding surface to increase so that an initial temperature equilibrium is achieved between the surface and the melting, and essentially instantaneously an interface temperature between them which is at least equal to the liquidus temperature of the melt, so that after solidification of the melting, a bond is achieved between the resulting second component and the first component (14) without a fusion of the bonding surfaces taking place. 2• Method as stated in claim 1, characterized in that a first component (14) is used which comprises an iron-based metal selected from mild steel, low-alloy steel and stainless steel. 3. Method as stated in claim 1 or claim 2, characterized in that the second component is selected from white cast iron, stainless steel types and cobalt-based alloys. 4. Method as stated in claim 3, characterized in that the first component (14) is selected from mild steel, alloy steel. including stainless steel, and cast iron including white chrome cast iron, and in that said second component is a white cast iron with from 2.0 to 5.0 wt% carbon and from 14 to 30 wt% chromium. 5. Method as stated in claim 4, characterized in that a second component is used where chromium is present in an amount of 25 to 30% by weight. 6. Method as stated in claim 4, characterized in that said white cast iron is used as the second component with a composition selected from the groups: (a) 2.4 to 3.6% by weight carbon, 0.5 to 1.5% by weight % manganese, 0 to 1.0 wt% silicon, 14 to 17 wt% chromium and 1.5 to 3.5 wt% molybdenum, the rest iron except for incidental impurities; (b) 2.3 to 3.0 wt% carbon, 0.5 to 1.5 wt% manganese, 1.0 wt% silicon, 23 to 30 wt% chromium and 0 to 1.5 wt% molybdenum, the balance iron except for incidental contaminants; (c) 2.5 to 4.5 wt% carbon, 2.5 to 3.5 wt% manganese, 0 to 1.0 wt% silicon, 25 to 29 wt% chromium and 0.5 to 1.5 wt% molybdenum, the rest iron except for incidental impurities; (d) 4.0 to 5.0 wt% carbon, 0 to 1.0 wt% manganese, 0.5 to 1.5 wt% silicon, 18 to 25 wt% chromium, 5.0 to 7.0 wt% molybdenum, 0.5 to 1.5% by weight vanadium, 5.0 to 10.0% by weight niobium and 1.0 to 5.0% by weight tungsten, the remainder iron except for incidental impurities; (e) 3.5 to 4.5 wt% carbon, 0 to 1.0 wt% manganese, 0.5 to 1.5 wt% silicon, 23 to 30 wt% chromium, 0.7 to 1.1 wt% molybdenum, 0.3 to 0.5 wt% vanadium, 7.0 to 9.0 wt% niobium and 0.2 to 0.5 wt% nickel, the remainder iron except for incidental impurities. 7. Method as stated in claim 3, characterized in that the first component (14) is selected from mild steel and alloy steel including stainless steel, and in that in said second component is an austenitic stainless steel having a composition selected from the groups: (a ) 0 to 0.08% by weight carbon, 18 to 21% by weight chromium, 10 to 12% by weight nickel, 2 to 3% by weight molybdenum and the remainder iron except for incidental impurities; and (b) 0 to 0.08 wt.% carbon, 18 to 21 wt.% chromium, 8 to 11 wt.% nickel and the remainder iron except for incidental impurities. 8. Method as stated in claim 3, characterized in that the first component (14) is selected from mild steel and alloy steel, and in that said second component is a cobalt-based alloy that has (Co, Cr) 7C3 carbides in a eutectic structure and a strain-hardenable matrix, obtained with a composition selected from the groups: (a) 28 to 31% by weight chromium, 3.5 to 5.5% by weight tungsten, 0 to 3.0% by weight each of iron and nickel, a 0 to 2.0% by weight each of manganese and silicon, 0 to 1.5% by weight molybdenum, 0.9 to 1.4% by weight carbon and the remainder cobalt except for incidental impurities; and (b) substantially 29% by weight chromium, 6.3% by weight tungsten, 2.9% by weight iron, 9.0% by weight nickel, 1.0% by weight carbon and the remainder cobalt except for incidental impurities. 9. Method as set forth in any of claims 1-8, characterized in that the first component (14) is at least partially preheated by flame heating applied in the mold cavity and which is maintained until after the casting of the forge has been completed. 10. Method as stated in claim 9, characterized in that the flame heating used provides reducing conditions in the mold cavity at least until the casting of the forge is complete. 11. Method as set forth in any of claims 1-8, characterized in that the first component (14) is at least partially preheated by flame heating applied to the same in a lower case component (12) of the mold, before placing an upper case part (20) of the mold, and that the flame heating ends before placing said upper case part (20) and before casting the metal. 12. Method as set forth in any of claims 1-11, characterized in that the flux is applied to the first component (14) as a slurry. 13. Method as set forth in any of claims 1-11, characterized in that the flux is applied to the first component (14) as a powder. 14. Method as set forth in any of claims 1-11, characterized in that the flux is applied by dipping the first component (14) in a melt of the flux. 15. Method as stated in claim 14, characterized in that the first component (14) is partially preheated by immersion in said flux melt before the component is placed in the mold cavity. 16. Method as set forth in any of claims 1-15, characterized in that the flux is used both to prevent oxidation of the surface of the first component (14) and to clean said surface of any oxide contamination. 17. Method as set forth in any of claims 1-16, characterized in that a metal is used for the first component (14) with a melting range that begins at a temperature higher than the liquidus temperature of the second component. 18. Method as set forth in any of claims 1-16, characterized in that a metal is used for the first component (14) with a melting range which is essentially the same as the melting range of the metal for the melt which provides the second component. 19. Composite metal article produced according to the method in claim 1, which article has a first component (14) and a second component, characterized in that the second component is cast against a surface of the first component (14), which article is characterized by a diffusion bond between said components obtained after solidification of the melt which provides said second component mainly without melting of said surface. 20. Composite article as stated in claim 19, characterized in that the first component (14) comprises an iron-based metal selected from mild steel, low-alloy steel and stainless steel. 21. Composite article as stated in claim 19 or claim 20, characterized in that said second component is selected from white cast iron, stainless steel, and cobalt-based alloys. 22. Composite article as stated in claim 19, characterized in that the first component (14) is selected from mild steel, alloy steel including stainless steel and cast iron including white chrome cast iron, and in that said second component is a white cast iron having from 2, 0 to 5.0% by weight carbon and chromium from 14 to 30% by weight. 23. Composite article as set forth in claim 22, characterized in that chromium is present in an amount of 25 to 30% by weight. 24. Composite article as set forth in claim 22, characterized in that said white cast iron has a composition selected from: (a) 2.4 to 3.6 wt% carbon, 0.5 to 1.5 wt% manganese, 0 to 1 .0 wt% silicon, 14 to 17 wt% chromium and 1.5 to 3.5 wt% molybdenum, the rest iron except for incidental impurities; (b) 2.3 to 3.0 wt% carbon, 0.5 to 1.5 wt% manganese, 0 to 1.0 wt% silicon, 23 to 30 wt% chromium and 0 to 1.5 wt% molybdenum, the rest iron except for incidental impurities; (c) 2.5 to 4.5 wt% carbon, 2.5 to 3.5 wt% manganese, 0 to 1.0 wt% silicon, 25 to 29 wt% chromium and 0.5 to 1.5 wt% molybdenum, the rest iron except for incidental impurities; (d) 4.0 to 5.0 wt% carbon, 0 to 1.0 wt% manganese, 0.5 to 1.5 wt% silicon, 18 to 25 wt% chromium, 5.0 to 7.0 wt% molybdenum, 0.5 to 1.5 wt% vanadium, 5.0 to 10.0 wt% niobium and 1.0 to 5.0 wt% tungsten, the balance iron except for incidental impurities; (e) 3.5 to 4.5 wt% carbon, 0 to 1.0 wt% manganese, 0.5 to 1.5 wt% silicon, 23 to 30 wt% chromium, 0.7 to 1.1 wt% molybdenum, 0.3 to 0.5 wt% vanadium, 7.0 to 9.0 wt% niobium and 0.2 to 0.5 wt% nickel, the remainder iron except for incidental impurities. 25. Composite article as stated in claim 21, characterized in that the first component (14) is selected from mild steel and alloy steel including stainless steel, and in that said second component is an austenitic stainless steel having a composition selected from: (a ) 0 to 0.08% by weight carbon, 18 to 21% by weight chromium, 10 to 12% by weight nickel, 2 to 3% by weight molybdenum and the remainder iron except for incidental impurities; and (b) 0 to 0.08% by weight. carbon., 18 to. 21 wt% chromium, 8 to 11 wt% nickel and the rest iron except for incidental impurities. 26. Composite article as set forth in claim 21, characterized in that said second component is a cobalt-based alloy having (Co,Cr) 7C3 carbides in a eutectic structure and a deformation-hardenable base mass, obtained with a composition selected from: (a) 28 to 31 wt% chromium, 3.5 to 5.5 wt% tungsten, a 0 to 3.0 wt% each of iron and nickel, a 0 to 2.0 wt% each of manganese and silicon, 0 to 1 .5 wt% molybdenum, 0.9 to 1.4 wt% carbon and the balance cobalt except for incidental impurities; and (b) substantially 29% by weight chromium, 6.3% by weight tungsten, 2.9% by weight iron, 9.0% by weight nickel, 1.0% by weight carbon and the remainder cobalt except for incidental impurities. 27. Composite article as defined in any of claims 19-2 6, characterized in that the metal in the first component (14) has a melting range that begins at a temperature higher than the liquidus temperature of the metal in the second component. 28. Composite article as claimed in any of claims 19-26, characterized in that the metal in the first component (14) has a melting range which is substantially the same as the melting range for the metal in the second component.